feat(server/m7-380): one-shot MBID backfill worker for existing libraries

Adds a boot-time goroutine that walks albums with NULL mbid, re-reads
tags from one track per album via dhowden/tag, and persists album + artist
MBIDs. Healed albums also get their cover_art_source='none' cleared so the
enricher's next batch retries the MBCAA fetch. Caps at 5000 albums per
boot to avoid stalling startup on huge libraries.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/db/dbq/albums.sql.go

@@ -347,6 +347,59 @@ func (q *Queries) ListAlbumsByGenre(ctx context.Context, arg ListAlbumsByGenrePa
 	return items, nil
 }
 
+const listAlbumsMissingMbidWithTrack = `-- name: ListAlbumsMissingMbidWithTrack :many
+SELECT a.id          AS album_id,
+       a.artist_id   AS artist_id,
+       a.title       AS title,
+       t.file_path   AS track_file_path
+  FROM albums a
+  JOIN LATERAL (
+    SELECT file_path
+      FROM tracks
+     WHERE album_id = a.id
+     ORDER BY disc_number NULLS LAST, track_number NULLS LAST, id
+     LIMIT 1
+  ) t ON true
+ WHERE a.mbid IS NULL
+ ORDER BY a.created_at ASC
+ LIMIT $1
+`
+
+type ListAlbumsMissingMbidWithTrackRow struct {
+	AlbumID       pgtype.UUID
+	ArtistID      pgtype.UUID
+	Title         string
+	TrackFilePath string
+}
+
+// One-shot MBID backfill: returns each album where mbid IS NULL alongside
+// one of its tracks' file_path so the worker can re-read tags. LIMIT
+// supplied by caller for batching/progress purposes.
+func (q *Queries) ListAlbumsMissingMbidWithTrack(ctx context.Context, limit int32) ([]ListAlbumsMissingMbidWithTrackRow, error) {
+	rows, err := q.db.Query(ctx, listAlbumsMissingMbidWithTrack, limit)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	var items []ListAlbumsMissingMbidWithTrackRow
+	for rows.Next() {
+		var i ListAlbumsMissingMbidWithTrackRow
+		if err := rows.Scan(
+			&i.AlbumID,
+			&i.ArtistID,
+			&i.Title,
+			&i.TrackFilePath,
+		); err != nil {
+			return nil, err
+		}
+		items = append(items, i)
+	}
+	if err := rows.Err(); err != nil {
+		return nil, err
+	}
+	return items, nil
+}
+
 const listAlbumsNewest = `-- name: ListAlbumsNewest :many
 SELECT id, title, sort_title, artist_id, release_date, mbid, cover_art_path, created_at, updated_at, cover_art_source FROM albums ORDER BY created_at DESC LIMIT $1 OFFSET $2
 `

internal/db/dbq/covers.sql.go

@@ -25,6 +25,20 @@ func (q *Queries) ClearAlbumCover(ctx context.Context, id pgtype.UUID) error {
 	return err
 }
 
+const clearAlbumCoverNone = `-- name: ClearAlbumCoverNone :exec
+UPDATE albums
+   SET cover_art_source = NULL
+ WHERE id = $1 AND cover_art_source = 'none'
+`
+
+// M7 #380: when MBID backfill heals an album, clear its 'none' cover_art_source
+// back to NULL so the enricher's next batch retries the MBCAA fetch.
+// Idempotent — only updates rows currently stamped 'none'.
+func (q *Queries) ClearAlbumCoverNone(ctx context.Context, id pgtype.UUID) error {
+	_, err := q.db.Exec(ctx, clearAlbumCoverNone, id)
+	return err
+}
+
 const countAlbumCoverSources = `-- name: CountAlbumCoverSources :one
 SELECT
     COUNT(*) FILTER (WHERE cover_art_source IS NULL)     ::bigint AS untried,

internal/db/queries/albums.sql

@@ -104,3 +104,23 @@ UPDATE albums
    SET mbid = $2,
        updated_at = now()
  WHERE id = $1 AND mbid IS NULL;
+
+-- name: ListAlbumsMissingMbidWithTrack :many
+-- One-shot MBID backfill: returns each album where mbid IS NULL alongside
+-- one of its tracks' file_path so the worker can re-read tags. LIMIT
+-- supplied by caller for batching/progress purposes.
+SELECT a.id          AS album_id,
+       a.artist_id   AS artist_id,
+       a.title       AS title,
+       t.file_path   AS track_file_path
+  FROM albums a
+  JOIN LATERAL (
+    SELECT file_path
+      FROM tracks
+     WHERE album_id = a.id
+     ORDER BY disc_number NULLS LAST, track_number NULLS LAST, id
+     LIMIT 1
+  ) t ON true
+ WHERE a.mbid IS NULL
+ ORDER BY a.created_at ASC
+ LIMIT $1;

internal/db/queries/covers.sql

@@ -44,6 +44,14 @@ UPDATE albums
        cover_art_source = NULL
  WHERE id = $1;
 
+-- name: ClearAlbumCoverNone :exec
+-- M7 #380: when MBID backfill heals an album, clear its 'none' cover_art_source
+-- back to NULL so the enricher's next batch retries the MBCAA fetch.
+-- Idempotent — only updates rows currently stamped 'none'.
+UPDATE albums
+   SET cover_art_source = NULL
+ WHERE id = $1 AND cover_art_source = 'none';
+
 -- name: CountAlbumCoverSources :one
 -- Diagnostic for the admin overview: counts by source, including NULL.
 -- Returns one row with named tallies.